If you’ve been left staring at your shoes after yet another mysterious Metro slowdown, take it out on the older trains—when they finally roll into the station. Door and brake problems were the primary reasons for train delays last year, causing 56 percent of service stoppages, according to Metro’s annual Vital Signs Report.

Metro’s oldest railcars, the nearly four-decade-old 1000-series, were the subject of a brake repair campaign in 2014, according to the report. The fixes included “[replacing] brake lines to prevent leaks, [installing] new brake control valves to apply instant brake pressure, and [replacing] rusting air compressor control boxes,” the report states. While most brake-caused delays don’t make the news, an apparent release failure by brake pads on a train pulling into the Dupont Circle station last week produced smoke and caused a brief evacuation. The train involved in Friday’s incident was a newer 5000-series model first introduced in 2001.

Cars from three out of Metro’s five series failed to reach the agency’s target distance of 60,000 mean miles between issues last year. The 1000-series cars managed an average of 57,228 miles between delays, while those of the 6000-series, the newest cars in service, went more than twice that distance without a delay. That shabby stat pales in comparison to those from the 23-year-old 4000-series models, which only covered 24,949 mean miles between delays. The series experienced a high number of door issues, which Metro attempted to rectify by replacing a part the agency says was failing prematurely.

“It has been well known for some time that the 4000s are much less reliable than others,” says Metro spokesman Dan Stessel. “The good news is that the 4000s are our smallest fleet. Normally the 4000s would be due for what is known as a ‘midlife overhaul’ right about now, and we are not even bothering with that—we’re going to scrap them outright.”

Meanwhile, the rollout of the intended replacement for the 1000-series and 4000-series, the 7000-series, has been delayed due to a dispute with an outside safety overseer. The rollout of the new cars, originally scheduled for last month, is now expected “within a matter of weeks,” Stessel says.

“Once the first train enters service, the new cars will be delivered at a rate of about 10 to 12 per month,” he says. “It will still take some time before the last 1000-series and 4000-series are off the road.” Once introduced, the new black-and-steel cars will run on every Metro line.

Photo by Darrow Montgomery